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  4. For what distance is ray optics a good approximation when the aperture is 4 mm wide and the wavelength is 500 nm?

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Q. For what distance is ray optics a good approximation when the aperture is $4 \,mm$ wide and the wavelength is $500 \,nm$?

Delhi UMET/DPMTDelhi UMET/DPMT 2009Ray Optics and Optical Instruments

Solution:

For $ Z_F $ distance ray optics is a good approximation.
$ Z_F=\frac{a^2}{\lambda} $
Here, $ a = 4 \times 10^{-3}m, \lambda = 5 \times 10^{-7} m $
$ \therefore Z_f=\frac{16 \times10^{-6} }{5 \times10^{-7}}=32\, m $